1. The Field of the Invention
This invention relates generally to systems and techniques for performing baselining and mapping of wires and cables. More specifically, the invention teaches how to utilize the principles of frequency domain reflectometry to perform baselining and mapping of single and multiple wires and cables, the baselining to be used as a comparison when the wires are tested at a later date to determine if the wires or cables have been damaged, and the mapping to be performed in order to determine the present configuration of a single wire or cable, or a network or tree of wires or cables.
2. Background of the Invention
The benefits of being able to test wires and cables (hereinafter to be referred to as a cable) are many. Some reasons are obvious. For example, cables are used in many pieces of equipment that can have catastrophic results if the equipment fails. A good example of this is an airliner. However, the consequences of non-performance do not have to be so dire in order to see that benefits are still to be gained. For example, cables are used in many locations where they are difficult to reach, such as in the infrastructure of buildings and homes. Essentially, in many cases it is simply not practical to remove cable for testing, especially when this action can cause more damage than it prevents.
Given that the need for cable testing is important and in some cases imperative, the question is how to perform accurate testing that is practical, meaning relatively inexpensive and at a reasonable cost. The prior art describes various techniques for performing cable testing. One such technique is time domain reflectometry (TDR). TDR is performed by sending an electrical pulse down a cable, and then receiving a reflected pulse. By analyzing the reflected pulse, it is possible to determine cable length, the type of load on the cable, and the location of open or short circuits.
One of the main disadvantages of TDR is that the equipment required to perform time analysis of a reflected signal is expensive and often bulky. These factors of cost and size can be critically important. A less costly and bulky system can be used in more places, more often, and can result in great savings in money spent on performing maintenance functions, and by replacing equipment before failure. But more importantly, the greatest benefit may be the saving of lives.
Consider again the airline industry. Miles and miles of cabling inside an airplane is extremely difficult to reach and test. If the cabling is removed for testing, the cabling can be damaged where no damage existed before. Thus, testing can result in more harm than good when cabling must be moved to gain access. But the nature of an airplane simply makes access with bulky testing equipment difficult. In addition, if the electronics for testing cables could remain in situ, then testing could be automated and used routinely before or after flight, or at any other time that testing was requested. This can be accomplished at this time only with smaller, less expensive systems such as provided by frequency domain reflectometry.
It is noted that TDR is not the only prior art technique available for cable testing. In standing wave reflectometry (SWR), a signal is transmitted and a reflected signal is received at a directional coupler. The system then measures the magnitude of the reflected signal. A short circuit, an open circuit, and the depth of a null gives the same information as TDR. However, this technique is generally less accurate and nearly as expensive as TDR.
It is worth noting that the prior art sometimes refers to an FDR cable testing system. However, upon closer inspection, the system being described is actually an SWR system as described above.
The FDR system to be described in this document is cable of very specific determination of cable characteristics. These characteristics include length, impedance (which is characterized as an open or short circuit condition), the location of an open or short circuit, capacitance, inductance, and resistance. However, for diagnostic purposes, it would be advantageous to also use the FDR system for baselining. Baselining is defined here as taking FDR measurements of a cable that is known to be in working order. Accordingly, it would be an advantage over the prior art to utilize FDR baseline measurements in order to perform testing of cables because the FDR system is relatively smaller and therefore usable in more locations that are otherwise more difficult to reach with state of the art cable testing equipment. It would be another advantage to provide a system that would be less costly because of the nature of the components utilized therein. It would be another advantage to provide a system that is more likely to be used because it is not as difficult to use as the prior art cable testing equipment, and can be automated for regular testing even by unskilled personnel.
The technology being applied to the problem of cable testing by the present invention has not previously been used for this purpose. Specifically, frequency domain reflectometry (FDR) is typically used in radar applications. FDR is based on single frequency radar or stepped frequency radar. It was utilized in a free-space environment where antennas are used to transmit and receive a radar signal. Thus, the results produced when used for cable testing were surprising to those skilled in the art.
It would also be an advantage over the prior art to utilize an FDR system for mapping. Generally, the layout of existing cabling is known. For example, a building will have wiring diagrams to describe the location and path of wiring. However, records may be lost. Consider also the situation where cabling may be modified without the knowledge or authorization of those who control it. For example, an illegal connection might be made to a television cable. Accordingly, it would be advantageous to be able to use the FDR system to determine the physical or structural layout of a cable in order to determine where all connections are being made.